COLLISIONS BETWEEN A WHITE-DWARF AND A MAIN-SEQUENCE STAR .2. SIMULATIONS USING MULTIPLE-NESTED REFINED GRIDS

We investigate the three-dimensional hydrodynamic evolution of a collision between a 0.5 MD white dwarf (gravitating point-mass) and a 0.5 M, main-sequence star (polytrope with index n = 1.5). Self-gravity is included by calculating the potential of matter on an equidistant CARTESIAN grid (32(3) and 64(3)) by fast FOURIER transforms. The hydrodynamics is modeled by the 'Piecewise Parabolic Method' (PPM). The initial separation of the stars is two stellar radii and they approach each other on parabolic orbits. No energy sources (nuclear burning) or sinks (radiation, conduction) are included. The resolution in the vicinity of the white dwarf is increased by multiply nesting refined grids around the white dwarf. This allows us to follow the evolution of central, off-center and grazing collisions for up to 20 h. The grazing encounter produces a bound orbit in which the white dwarf accretes matter from the deformed star, in a second collision the main-sequence star is finally disrupted and forms a disk. Only little mass (approximately 15% for the grazing collisions; up to 40% for the central collisions) is expelled from the system.